1. Field of the Invention
The present invention relates to the field of plant breeding and, more specifically, to the development of downy mildew-resistant lettuce having elite agronomic traits.
2. Description of Related Art
Cultivated lettuce, Lactuca sativa, is a temperate annual or biennial plant most often grown as a leaf vegetable. Lettuce belongs to the family Asteraceae (or Compositae). Other members of this family include endive, chicory, artichoke, sunflower and safflower. It is closely related to common wild lettuce or prickly lettuce (L. serriola) and less closely related to two other wild lettuces (L. saligna and L. virosa). Cultivated lettuce and sunflower are the best genetically characterized members of this family. Four principal types of cultivated lettuce include crisphead (mostly iceberg), romaine (cos), leaf and butterhead. Each of these basic groups is comprised of numerous cultivars, each characterized by its own particular morphology, cultural adaptations, and disease resistance. Lettuce cultivars are susceptible to a number of diseases such as Downy Mildew, Sclerotinia Rot, Botrytis Rot, Corky Root Rot, Bacterial leafspot of lettuce, caused by Xanthomonas campestris pv. vitians, and lettuce mosaic virus, among others. Among the most important fungal diseases of lettuce is Downy Mildew, caused by Bremia lactucae (Regel). L. saligna displays quantitative resistance to Bremia lactucae such that it is generally considered to be a non-host plant of this oomycete, and has been studied as a potential source of genetic resistance to this disease.